Field of the Invention
The present invention relates to a developing apparatus and an image forming apparatus in which an electrophotographic system or an electrostatic recording system is employed to form an image.
Description of the Related Art
Conventionally, many image forming apparatuses employing the electrophotographic system or the electrostatic recording system use a two-component developer in which toner and carrier are mixed. In particular, a full-color image forming apparatus which forms a full-color image by the electrophotographic system uses the two-component developer from a viewpoint of a coloring property and a color mixing property.
A development method using the two-component developer is a method in which the toner is charged by frictional electrification between the carrier and the toner and the charged toner is attached to a latent image in an electrostatic manner to form an image.
In the development method using the two-component developer, it is important to stabilize a toner charging amount (hereinafter, referred to as a “tribo”) in order to form an image while stabilizing the density within a slight variation. Therefore, it is necessary to make a density distribution of the toner in the developing apparatus uniformed.
In general, the tribo is easily influenced by the toner density. Further, when the toner density is decreased, an absolute value of the tribo tends to be increased, and when the toner density is increased, the absolute value of the tribo tends to be decreased.
Therefore, in a case where the toner is consumed during a developing operation and the toner density of the developer is lowered, the developing apparatus of the related art performs control such that the toner corresponding to the amount of the consumed toner is replenished and agitated to keep a constant amount of the toner density.
However, when the developer is used for a long time, the developer collected from a developing sleeve into the developing apparatus is not sufficiently mixed with the developer already in the developing apparatus during the developing operation, so that the toner density is unevenly distributed in part. Then, the collected developer is supplied to the developing sleeve again, and as a result there causes a problem in that the density is lowered.
As a countermeasure of the above problem, there is proposed a configuration in which when the toner is consumed during the developing operation, the developer comes to be in a state of low toner density, and thus the collected developer is not supplied to the developing sleeve at once. Specifically, a supplying chamber to supply the developer to the developing sleeve and a collecting chamber to collect the developer from the developing sleeve are divided. Therefore, the developer just collected to the developing apparatus is prevented from being supplied to the developing sleeve at once (see Japanese Patent Laid-Open No. 1993-333691). FIG. 10 is a diagram for describing a conventional example.
As illustrated in FIG. 10, the developing apparatus is provided with the supplying chamber in the upper portion of the developing apparatus and the collecting chamber in the lower portion thereof. The developer is circulated between the collecting chamber and the supplying chamber using a first conveying screw 101 and a second conveying screw 102 which are contrary to each other in the conveying direction. Then, while circulating the developer, the developer is supplied from the supplying chamber in the upper portion to the developing sleeve to perform development on a photosensitive member. On the other hand, after the development, the developer is collected from the developing sleeve into the collecting chamber in the lower portion. With this configuration, the developer of which the toner density is lowered after the development is not directly supplied to the developing sleeve. Therefore, the problem in that the toner density becomes partially uneven or the density is decreased is alleviated.
However, even in a case where such a developing apparatus is used, when a lot of toner is consumed such as a case where an image is printed at a high rate, there still remains the problem in that the uneven toner density is partially generated or the density is decreased.
The following reasons can be considered. First, the toner replenished into the developer and the developer collected from the developing sleeve are joined in the collecting chamber, agitated and conveyed. Herein, in an axial direction of the developing sleeve, in an area on a downstream side in the conveying direction of the collecting chamber near a communication portion through which the developer is transferred from the collecting chamber toward the supplying chamber, the developer collected from a developing portion and fallen into the collecting chamber is relatively readily transferred to the supplying chamber. Then, the developer fallen on an inner side of the collecting chamber is not sufficiently agitated with the developer replenished with the toner and thus not sufficiently mixed with each other, and in this state the developer is easily transferred to the supplying chamber.
The above problem is not significant in the case of a low printing rate, but when the toner is not sufficiently agitated in the case of a high printing rate, the developer having an uneven density is transferred to the supplying chamber. Therefore, there is a possibility to supply such developer to the developing sleeve.
Further, the developer is likely to remain on the inner side near the communication portion which transfers the developer from the collecting chamber to the supplying chamber on the downstream side of the second conveying screw 102. Then, when the amount of developer is increased or when the fluidity of the developer is lowered, there is a concern that the developer overflows to the outside of a development device.
As a countermeasure of the above problem, there is proposed a developing apparatus provided with a third conveying screw 103 which conveys the developer in a direction opposite to a conveying direction of the second conveying screw 102 in addition to the second conveying screw 102 in the collecting chamber (see Japanese Patent Laid-Open No. 1994-051634). FIG. 11 is a diagram for describing a conventional example.
As illustrated in FIG. 11, since the third conveying screw 103 is provided, the developer remained on an inner side near the communication portion which conveys the developer from the collecting chamber toward the supplying chamber can be pushed in a direction opposite to the conveying direction of the second conveying screw 102. Therefore, the developer bearing surface can be evenly formed, and the fallen developer can be more effectively agitated.
However, as in the configuration disclosed in Japanese Patent Laid-Open No. 1994-051634, in a configuration in which the developer overflowing toward the third conveying screw in the developer remained in the communication portion from the collecting chamber toward the supplying chamber is evenly conveyed in a longitudinal direction to be pushed, the developer in the downstream end portion of the third conveying screw may be leaked.
In other words, a conveyance force of the third conveying screw is increased in order to form the developer bearing surface evenly. However, on the other hand, when the conveyance force of the third conveying screw is too much increased, the developer is excessively conveyed to raise a developer pressure (a pressure applied onto the developer) in the downstream end portion of the third conveying screw. As a result, as described above, the leakage of the developer may occur.
Further, there is a configuration in which the blade of the third conveying screw is not provided in the whole area but up to the midway in the longitudinal direction in order to increase an assembly property. In this configuration, since the developer is not conveyed up to the downstream end portion, the developer leakage from the end portion does not occur. However, the developer remains in a boundary portion of the blade of the third conveying screw, and the developer may overflow from the position of the boundary portion.